The present circuit arrangement and method are particularly adapted for use in a transponder or in a remote sensor that has a transponder for responding to an interrogation transmitted through an electromagnetic field. In this context the term “transponder” is intended to encompass any apparatus that receives its operating energy from an electromagnetic field. For this purpose the apparatus has a voltage converter for generating a function voltage from an operating voltage for performing a function sequence. The term “function sequence” is intended to include a single function.
In connection with radio frequency identification methods ever larger response ranges are required of the respective identification devices. This applies particularly to methods and devices in the RFID-field (Radio Frequency Identification Devices) using so-called passive transponders which regularly must sustain their standby operation with a small electrical power while deriving their operational power from a base station which transmits the operation electrical power to the passive transponder through an electromagnetic field.
In this context the term “response range” or “function range” is intended to mean a distance between the base station and the transponder over which distance the transponder can return a useful signal to the base station in response to an interrogation from the base station. In order to transmit such a useful signal the transponder must be capable of deriving from the electromagnetic field a function voltage that is distinctly higher than its normal operating or standby voltage. Additionally, the higher function voltage must be available for a time duration sufficient for performing a respective function sequence. Further, this function voltage must be available with certainty, at least for the required time duration.
Such a function sequence may, for example, be a programming operation, more specifically, writing into a suitable memory or reading from a suitable memory such as an EEPROM (Electronically Erasable Programmable Read Only Memory). In the case of a remote sensor which encounters the same problems, the function may, for example, be a measuring operation. For example, programming voltages in conventional EEPROMS are within the range of 12 to 14V compared to a standby voltage, also referred to as operating voltage, in the range of 1 to 3V. In this context the higher function voltages must be available for a time duration of a few milliseconds in order to successfully and completely perform a function sequence.
In connection with conventional passive transponders the programming voltage, also referred to as function voltage is generated with a multistage voltage converter or a so-called multistage charge pump. Charge pumps as such are well known. The generation of the higher voltage or rather the result of the generation is referred to as “run-up” voltage. Conventionally, a predetermined waiting period follows the voltage run-up. The waiting period is based on the assumption that a sufficient function voltage has been present during the waiting period for properly concluding the intended function sequence. Conventional devices of this type do not permit any checking of the function sequence itself or the result of the function sequence. It is a particular disadvantage of conventional devices that a function duration, for example a programming duration, is fixed independently of the actual time needed for such programming.
It is further known in connection with devices of this type that the operating voltage is measured at the time when a function cycle is taking place. This measuring of the operating voltage is done in order to take into account load variations, for example due to a delayed or prolonged voltage run-up, due to an increased switch-on current and/or due to parasitic currents in the saturation phase. The measured operating voltage values are then compared with a reference value, whereby positive comparing results are counted and whereby, following a passing of a predetermined time, a further comparing is performed with a further reference value. If a function duration is shorter than the further reference value, the conclusion is reached that the respective function sequence was not successful. These conventional approaches for solving the problem of achieving an increased response range have particularly the disadvantage that the measuring of the operating voltage merely is an indirect measurement of the function voltage, whereby only a breakdown of the operating voltage provides a certain reference point regarding the function voltage. Another disadvantage of conventional devices is seen in that the control is a pure open loop control in which the function time duration is again determined independently of the actual run-up time. Thus, the function time duration is not adaptable to concrete actual requirements such as an increased programming range or rather response range.
Particularly with regard to the last mentioned requirement of an increased programming or response range it is seen as a disadvantage in known devices that in connection with high impedance energy sources the operating voltage can break down in response to an increased switch-on voltage. Such voltage break down can happen, for example, if the impedance of a voltage converter for producing a direct voltage out of the alternating voltage energy taken from the electromagnetic field, is too large. Such a situation leads to a so-called “power-on-reset” (POR), whereby the response range is further reduced. Conventional voltage multipliers have the same effect. Such voltage multipliers comprise a multitude of transistor switches which lead to losses due to respective voltage drops across these switches.